Prior to setting forth the background of the invention, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “thermoplastic material” or “thermoplastic” as used herein in this application, relates to a plastic that melts to a liquid when heated and freezes to a brittle, very glassy state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). Thermoplastic polymers can be re-melted and remolded. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene.
The term “thermosetting plastic”, “thermoset” or “thermoset material” as used herein in this application, relates to polymer materials that irreversibly cure form. The cure may be done through heat (generally above 200 degrees Celsius), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing.
Uncontrolled reheating of the material results in reaching the decomposition temperature before the melting point is obtained. Therefore, a thermoset material cannot be melted and re-shaped after it is cured. Thermoset materials are generally stronger than thermoplastic materials due to a three-dimensional network of bonds, and are also better suited to high-temperature applications up to the decomposition temperature of the material.
The term “flexographic printing” or “flexography” as used herein in this application, relates to a printing method that involves creating a positive mirrored master of the required image as a three-dimensional relief in a rubber or polymer material. A measured amount of ink is deposited upon the surface of the printing plate (or printing cylinder) using an engraved anilox roll whose texture holds a specific amount of ink. The print surface then rotates, contacting the print material which transfers the ink.
Flexographic printing is a technologically challenging task. Minute differences in plate thickness, consistency and even surface energy can cause large deviations in ink transfer uniformity. Such deviations may result in poor quality printing and non smooth printed areas. The thickness tolerances that are commonplace in the flexographic printing ranges from −13 to +13 microns (μm). This value was chosen since these are the tolerances available for printing drums, gears and cogwheels.
Manufacturing printing plates to a thickness uniformity of ±13 μm is not an easy task. Conventional and Laser Ablation Mask System (LAMS) plates are made of a thermoplastic material which gives leverage to fabrication with systems such as extrusion followed by precise calendering of the sheet that is to be cut into plates. Plates that are imaged by direct engraving, however, have to be thermoset. The plate material has to be thermoset to overcome the problem of edge melting by the engraving laser as well as to give chemical and solvent resistance and mechanical strength. Plate materials that are used in the direct engraving industry include rubber, crosslinked photopolymer and silicone based rubbers. Rubber plates are made by calendering the sheet material with heated rollers to induce vulcanization. Photopolymer material is exposed to UV to induce crosslinking, introducing chemical and solvent resistance. Since photopolymer plates are transparent to UV and near IR, these plates are engraved exclusively by CO2 lasers. Some of the direct engraving plates on the market are ground to give the thickness uniformity prior to imaging.
Fabricating thermoset materials in sheet form to a tolerance of ±13 μm from liquid or paste poses even more challenges. Thermoset pastes are rarely extruded for fear of clogging the extruder. Special precautions must be taken to avoid sticking of the material to the calendering rollers, if such a system is chosen. In fact, many manufacturers of extrusion equipment avoid experimenting with thermoset materials because of the difficulties involved in cleaning out the extruders. Casting the materials does not give tolerances of the above mentioned figures over standard plate sizes.